Resonant Frequency

Discussion in 'The Projects Forum' started by Engineer_tech, Oct 24, 2014.

  1. Engineer_tech

    Thread Starter Member

    Sep 19, 2014
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    Hi
    i need some help regarding resonant frequency
    i have a metal bar with magnet attached to on of its end.can i find the resonance frequency of the metal bar by driving a coil with function generator(sine wave) and placing the bar within the magnetic field range of coil.
    how can i setup the system to find the resonance frequency of the metal bar having magnet attached to it.
    Thank You for putting your expert input
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    thats only acoustic or mechanical resonance, like a tuning fork, not electrical resonance.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The proposed system should work, provided the fixed end of the bar has a non-resonant mount.
     
  4. Engineer_tech

    Thread Starter Member

    Sep 19, 2014
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    non resonant mount means the other end has to be mounted fixed.the metal bar i am using is L-shaped with the L side being mounted and other end has magnet in it.
    and lets say i start from 100 HZ frequency and keep on changing it til i am able to vibrate it.should i keep on changing or i need to wait for some time on certain frequency and then increase it and wait again?
     
  5. crutschow

    Expert

    Mar 14, 2008
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    If you slowly change the frequency you will observe the point of maximum vibration which is the bar resonant frequency. Once you pass the resonant frequency you can slowly change the frequency up and down to find the maximum point.
     
  6. Engineer_tech

    Thread Starter Member

    Sep 19, 2014
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    i started from few hz to 10 Khz but i the metal bar did not vibrate.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Perhaps the magnetic field strength of the coil is not enough to cause discernable vibration. How are you sensing vibration?
     
  8. Engineer_tech

    Thread Starter Member

    Sep 19, 2014
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    well for now all i setup is that i connected function generator sine wave (5v p-p) to a coil and placed the metal bar with magnet on one end near to it.the other end of L-shaped rod is mounted to a holder(metal holder to hold the bar).
    i have a question here can i scope the current or voltage of the coil?(when i connect probes to both ends of the coil(being driven by function generator) i am not getting any pulse on the scope.?
    -can i calculate the voltage being induced by the coil.?as my understanding is that the coil will produce magnetic field lined in its surrounding which will induce voltage in any other coil in its field?
    is there any modifications needed?
     
  9. crutschow

    Expert

    Mar 14, 2008
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    You may not see any voltage because the function generator cannot drive the low coil impedance. If so, you need an buffer amp to drive the coil.
    What is the resistance of the coil?
     
  10. Johann

    Senior Member

    Nov 27, 2006
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    How about using a crystal microphone insert as a sensor on the bar? Feed the microphone signal into a wide band amplifier and observe the output on an oscilloscope.
     
  11. Engineer_tech

    Thread Starter Member

    Sep 19, 2014
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    i think the low resistance of the coil might be the reason.the coil has a resistance of 4-5 ohm and i set the load impedance in function generator as 5 ohms which gives me a max peak to peak voltage of 1.8
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What size, thickness, springiness does the bar have? About how much force do you need to apply to the end to deflect it, say, 1mm?
     
  13. Engineer_tech

    Thread Starter Member

    Sep 19, 2014
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    bar is thick i dont have idea about the force required to deflect it(not stretchable with hand).the bar is about 2 inches long and around 5 mm thick
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Hmm. Then, as Crutschow said in post #9, looks like you're going to need an amplifier to drive the coil. Even then the vibration won't be detectable by eye, so you'll also need a sensing transducer and associated second amplifier.
     
  15. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Does it "ring" if you tap it lightly with a solid object - e.g. "small hammer".
    Also, there may be more than one resonance condition.
     
  16. Engineer_tech

    Thread Starter Member

    Sep 19, 2014
    31
    1
    Thank You all for the input and expert advise
    i have been able to make the rod vibrate by just increasing the number of turns in the coil i was able to create magnetic field strength strong enough to make the bar vibrate.
     
  17. BR-549

    Well-Known Member

    Sep 22, 2013
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    I doubt that.
    2" long and 5 mm thick. At what frequency is it vibrating? Is it vibrating at more than 1 frequency?
    You might move the magnet with the coil..........but I doubt the bar is vibrating.
    I don't think it is in resonance acoustically or electrically.
     
  18. Engineer_tech

    Thread Starter Member

    Sep 19, 2014
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    well all i did was i put some magnet on to a rod the magnet is embedded inside the rod and cant move. then i drove the coil with sine Wave.
    i mean i cant see the rod vibrating but it does make sound on a certain frequency
     
  19. BR-549

    Well-Known Member

    Sep 22, 2013
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    Engineer tech....That's quite something. Congratulations. I would really like to see your setup and listen to that sound. What frequency does it sound on?
    I always learn new things here.
     
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